Fuel cell is an electric power generating device having an anode (fuel electrode) positioned on one side and a cathode (air electrode) on the other side with an electrolyte interposed between them, in which fuel in an anode gas flowing on the anode surface and an oxidizer in a cathode gas flowing on the cathode surface are electrochemically reacted with the aid of the electrolyte (cell reaction) to generate electricity. Solid oxide fuel cell is a type of fuel cell using an oxygen ion conducting solid electrolyte, and as it is operated at a high temperature of 700 to 1,000° C., it has an advantage of being capable of conducting a hydrocarbon reforming reaction using the anode as a catalyst. The electrochemical reaction that occurs at the cathode of the solid oxide fuel cell is expressed by the formula (1) shown below, and the electrochemical reactions taking place at the anode are expressed by the formulae (2) and (3). The hydrocarbon reforming reactions are also expressed by the formulae (4) and (5).O2+4e−→2O2−  (1)H2+O2−→H2O+2e−  (2)CO+O2−→CO2+2e−  (3)CnH2m+nH2O→nCO+(n+m)H2  (4)CnH2m+nCO2→2nCO+mH2  (5)
Hydrocarbons, when heated to a high temperature of 500° C. or above, are cracked to cause carbon deposition. When such carbon deposition occurs on the anode, electrode performance deteriorates. In order to prevent such carbon deposition, water vapor and/or carbon dioxide are added so that the ratio of oxygen atom to carbon atom (hereinafter referred to as O/C ratio) in the anode gas will be kept at 2 or above. As seen from the formulae (4) and (5), addition of water vapor and/or carbon dioxide to the hydrocarbon is necessary for its reforming, too.
The electromotive force of the solid oxide fuel cells is decided by the ratio of the partial pressure of oxygen in the cathode gas to the partial pressure of oxygen in the anode gas. The oxygen concentration in the anode gas is decided by the chemical equilibrium expressed by the following formulae (6)-(8):2H2+O22H2O  (6)2CO+O22CO2  (7)CnH2m+(n+m/2)O2nCO2+mH2O  (8)
The electromotive force attenuates when the ratio of the concentration of fuel such as hydrogen, carbon monoxide and hydrocarbon to the concentration of water vapor and/or carbon dioxide lowers. Therefore, supply of fuel in admixture with water vapor and/or carbon dioxide leads to a reduction of electromotive force. It is thus desirable, in terms of enhancement of electromotive force, to minimize incorporation of water vapor and carbon dioxide in the fuel.
In the solid oxide fuel cells, the anode gas flows from the inlet toward the outlet, and as it passes along the anode surface, the fuel concentration in the anode gas decreases due to the electrochemical reactions between the fuel and the oxidizer while the concentration of water vapor and/or carbon dioxide increases. Because of this compositional distribution of the anode gas, the electromotive force lessens in accordance with the flow of the anode gas from the inlet toward the outlet. Consequently, the current density falls down at the electrode near the anode gas outlet where the electromotive force is low, and the current is concentrated at the electrode near the anode gas inlet where the electromotive force is high. Voltage drop caused by this concentration of current density has been blamed for the fall of power generating efficiency of the cells. As a means for curving the decrease of fuel concentration in the anode gas, it has been known to uniformly distribute the fuel of the same composition in the whole area of the anode, as for instance disclosed in Patent Document 1.    Patent Document 1: JP-A-8-203552